Mobile IP (v4/v6), also indicated as MIPv4 and MIPv6 enables a mobile node (MN) to register its temporary location indicated by a care-of-address (CoA) to its Home Agent (HA). In MIP the HA then keeps a mapping (also called a binding) between the MN's permanent address, otherwise called Home Address (HoA), and the registered CoA so that packets for that MN can be redirected to its current location using IP encapsulation techniques (tunneling).
The CoA used by a MN can be an address that belongs to a Foreign Agent (FA) when MIPv4 is used or, in MIPv4 and MIPv6, it can be a temporarily allocated address to the MN itself in which case is called a collocated care-of-address (CCoA).
The concepts and solutions described here are applicable to both MIPv4 and MIP unless otherwise mentioned.
MIPv4/v6 also has a feature called reverse tunneling. This ensures that all uplink traffic from the MN goes via the HA before its final destination. The traffic is essentially tunnelled back to the HA either by the MN itself or by the FA the MN is connected to. Similarly as before, the HA will not accept reverse tunnelled packets from a given CoA or CCoA unless the MN registers that CoA/CCoA with it.
In Mobile IP the home subnet is the location of the HA and is also where the MN is typically located. When a MN is on its home subnet, the MN responds to Address Resolution Protocol (ARP) requests for the HoA. When it is away from home, the HA instead uses proxy ARP to respond to ARP requests for the HoA of the MN so that packets for the MN are routed towards and by the HA towards the current CoA. When a MN returns home, the HA and the MN send gratuitous ARP signals to update all the ARP caches to inform them that the MN is now home and that the link-layer address for the HoA is now that of the MN and not the HA. If the MN is not at home, and the HA does not have a current CoA binding for the MN, then both the HA and the absent MN will ignore incoming packets which will blindly be dropped on the subnet. The AR processing is described in section 4.6 of IETF RFC 3220. In mobility systems, such as in 3G cellular or 802.11, especially when dynamic addressing is employed, the MN typically does not have a home subnet and there is never a MN available to respond to ARP requests in the absence of a current CoA binding in the HoA, maintained by the MN.
Additionally, in mobility systems, the MN may be absent from the system for a number of reasons. The MN could be switched off, unreachable in a disconnected part of the Internet fabric (a private domain), it could be in various forms of power-saving sleep states, or could simply not wish to be reachable on a specific HoA (privacy, on-leave etc). Therefore, when the MN is absent and not maintaining its CoA binding, incoming packets for that HoA will simply be dropped on the local subnet.